Optical recording medium

ABSTRACT

A multilayer optical recording medium in which, as a recording layer is located farther from a surface of incidence of a light beam for reading, the amount of light that reaches the surface of incidence after being reflected off the recording layer is smaller.

TECHNICAL FIELD

The present invention relates to an optical recording medium such as anoptical disk with a large number of stacked recording layers.

BACKGROUND ART

In a multilayer optical disk with a large number of stacked recordinglayers, the reflectance of each recording layer is generally designedsuch that the amounts of light reflected back from the respectiverecording layers are the same on a surface of incidence of a light beam.Meanwhile, the multilayer optical disk suffers from multiple reflection.With reference, for example, to a multilayer optical disk with eightstacked recording layers L0 to L7 shown in FIG. 1, for playback of therecording layer L0 as a target layer located farthest from a surface ofincidence of a light beam of the optical disk, the light beam is focusedon the recording layer L0, and is reflected off the recording layer L0.At the same time, the light beam is also reflected off the recordinglayers L1, L2, and L3 near the recording layer L0. The light reflectedoff the recording layers L1, L2, and L3 forms first, second, and thirdconfocal spots on the back surfaces of the recording layers L2, L4, andL6, respectively. These spots generate interlayer crosstalk, which ishereinafter called multiple reflection CT.

There is a known conventional technique of reducing multiple reflectionCT (see Patent Literatures 1 to 4), in which every distance betweenmultiple recording layers of an optical disk is defined such that atleast some of distances between the adjacent recording layers differfrom each other.

-   [Patent Literature 1] Japanese Patent Kokai No. 2001-155380-   [Patent Literature 2] Japanese Patent Kokai No. 2006-40456-   [Patent Literature 3] Japanese Patent Kokai No. 2006-59433-   [Patent Literature 4] Japanese Patent Kokai No. 2006-252752

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Even the simplest technique requires two types of interlayer distances.This leads to complication of a manufacturing method of a disk, therebyreducing yield and increasing manufacturing cost. The complication isdescribed in detail next.

A technique of using film sheets, and a spin coat technique aregenerally employed to form intermediate layers of a multilayer disk. Thetechnique of using sheets realizes reduction in error of an interlayerdistance (shift from an established interlayer distance). Meanwhile,this technique requires sheets of several types in order to definemultiple interlayer distances, and a sheet type should be changed foreach recording layer, thereby reducing yield. In the spin coattechnique, an interlayer distance can be changed by adjusting parameterssuch as the amount of resin to be dropped and the number of revolutionsof a spindle. However, these parameters are generally adjusted veryseverely. Accordingly, in many cases, even slight change in asurrounding environment such as temperature and humidity generates alarge error of an interlayer distance. The parameters should suitably beadjusted to reduce an error of an interlayer distance. Further, theparameters should be adjusted several times if interlayer distances ofseveral types are to be defined. This leads to complication of amanufacturing method of a disk that leads to reduction in yield, therebyresulting in increase of manufacturing cost as described above.

If the foregoing multilayer disk has nonuniform interlayer distances, adrive for recording and playback of this disk should change the amountof movement of an objective lens, the amount of correction of sphericalaberration and the like for each recording layer when a jump is to bemade between layers, resulting in a complicated control logic.

The aforementioned problem is an example of a problem to be solved bythe invention. The invention is intended to provide an optical recordingmedium with a large number of recording layers the structure of which issimpler than a conventional structure, and which is capable of reducingmultiple reflection.

Means for Solving the Problem

An optical recording medium of the invention according to claim 1 is amultilayer optical recording medium with at least three stackedrecording layers. In this optical recording medium, as a recording layeris located farther from a surface of incidence of a light beam forreading, the amount of reflected light that reaches the surface ofincidence after being reflected off the recording layer is smaller.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

In the optical recording medium of the invention according to claim 1,the amount of light reflected back from a recording layer is smaller asthe recording layer is located farther from the surface of incidence.This realizes reduction in multiple reflection CT during playback of alayer located farther from the surface. Accordingly, a jitter value ofthe layer located farther from the surface is improved.

Embodiments

An example of the present invention will be described in detail withreference to the drawings.

FIG. 2 is a diagram showing the cross-sectional structure of an opticaldisk according to the present invention. The optical disk is a 20-layerdisk, and includes a substrate 1, recording layers L0 to L19 eachcomposed of a reflective film on which a pit sequence is formed, andintermediate layers 2 between the recording layers. The recording layersL0 to L19 are arranged in this order as viewed from the substrate 1. Theintermediate layers 2 made of an ultraviolet curable resin are placedbetween the recording layers L0 to L19. The recording layer (nearestlayer) L19 is placed through the intermediate layer 2 at a positionnearest a disk surface on which a laser beam impinges. The recordinglayers L0 to L19 are made of a dielectric such as Nb₂O₅ and TiO₂.

As shown in FIG. 3, the amounts of light reflected back from therecording layers L0 to L19 to the surface of incidence of beam light ofthe disk decreases in the order from the recording layer L19 to therecording layer L0.

The reflectance of each of the recording layers L0 to L19 is determinedto satisfy the following. As an example, the amounts of light reflectedback from the recording layers L19, L18, and L17 are 1.20%, 1.17%, and1.14% of the amount of the light entered the disk, respectively. Thatis, the amount of reflected light is changed in decrements of about0.03% (=0.6/19) for each recording layer. Further, the amount of lightreflected back from the recording layer L0 is half the amount of lightreflected back from the recording layer L19, namely is 0.06%.

The respective reflectances of the recording layers L19 to L0 are set to1.36%, 1.42%, . . . and 3.71% so that the amount of reflected light thatreaches a surface of incidence of a light beam after being reflected offa recording layer is smaller as the recording layer is located fartherfrom the surface of incidence of a light beam.

The reflectance of the surface of incidence of the disk is 4.0%, theabsorption factor of each intermediate layer is 2.2%, and interlayerdistances are uniformly 10 μm.

As shown in FIG. 4, the relationship between the thickness of thedielectric reflective films (in the case of Nb₂O₅) forming the recordinglayers L0 to L19 and a reflectance has such characteristics that areflectance increases with a greater film thickness if the filmthickness is equal to or smaller than 40 nm. It is assumed, for example,that the refractive indexes of Nb₂O₅ and an ultraviolet curable resinused for forming the intermediate layers 2 are 2.5 and 1.6,respectively. In this case, the thicknesses of the recording layers L19and L0 are about 7 nm and 12 nm, respectively. The wavelength of a lightbeam is set at 405 nm in FIG. 4. Accordingly, the respective thicknessesof the recording layers L0 to L19 decrease in the order from therecording layer L0 to the recording layer L19, so that the amounts oflight reflected back to the surface of incidence of the disk decrease inthe order from the recording layer L19 to the recording layer L0 asshown in FIG. 3.

The aforementioned optical disk of the present invention is formed on achemically strengthened substrate 11 (of an exemplary outer diameter φ120 mm and an exemplary inner diameter φ 15 mm) by using an ultravioletcurable resin 12. The thickness of the ultraviolet curable resin 12becomes an intermediate layer (represented by the aforementioned numeral2) that defines a distance between recording layers. Pits aretransferred to a surface of the ultraviolet curable resin 12, so thatthe ultraviolet curable resin 12 becomes a signal transfer layer. Adielectric (Nb₂O₅) 13 is deposited on a surface of the signal transferlayer by sputtering, thereby forming a recording layer. The amount oflight reflected back from each recording layer is determined for eachrecording layer by changing the thickness of each recording layer undera separately defined condition.

More specifically, an intermediate layer made of the ultraviolet curableresin 12 is formed by the spin technique concurrently signal transfer.As shown in FIG. 5( a), an appropriate amount of the ultraviolet curableresin 12 is dropped onto the substrate, and then a stamper 14 on whichpits are formed is placed thereon. The substrate 11 in this state iscaused to rotate at high speed to blow off excess portion of theultraviolet curable resin 12. As a result, an ultraviolet curable resinlayer 12 is formed that has a thickness determined by the number ofrotations and a duration of rotation. Next, ultraviolet rays are appliedto cure the ultraviolet curable resin 12 as shown in FIG. 5( b). Thestamper 14 is removed after the curing, and the dielectric 13 isdeposited to form a reflective film, thereby forming one recording layeras shown in FIG. 5( c). The ultraviolet curable resin 12 is furtherdropped onto the recording, and then the aforementioned steps arerepeated. A cover layer (not shown) is formed in a final step, therebyforming multilayer disk. Ultraviolet rays for curing an ultravioletcurable resin may be applied onto either the substrate 11 or the stamper14. If ultraviolet rays are to be applied to the stamper 14, the stamper14 should be made of a material that allows an ultraviolet ray to passtherethrough. If ultraviolet rays are to be applied to the substrate 11,the penetrating power of an ultraviolet ray decreases with an increasingnumber of recording layers. Accordingly, in this case, the intensity ofan ultraviolet ray, and the accumulated amount of light should beincreased.

A disk drive device intended for playback of the optical disk of thepresent invention includes a playback optical system, and a signalprocessing system.

As shown in FIG. 6, the playback optical system includes a light source21, a collimator lens 22, a beam splitter 23, an expander lens 24, anobjective lens 25, a condenser lens 26, and a detector 27. The signalprocessing system includes a laser driving circuit 30, a signalprocessing circuit 31, a controller 32, an objective lens drivingcircuit 33, a spherical aberration correcting element driving circuit34, and a memory 35. The optical disk mentioned above is represented bynumeral 20 in FIG. 6.

The light source 21 is a semiconductor laser element for emitting laserbeams. The collimator lens 22 converts laser beams emitted from thelight source 21 to parallel beams, and supplies the parallel beams tothe beam splitter 23. The beam splitter 23 transfers the parallel laserbeams supplied from the collimator lens 22 to the expander lens 24 asthey are. The expander lens 24 is a spherical aberration correctingelement, and has first and second correction lenses 24 a and 24 b. Thecorrection lenses 24 a and 24 b are driven by actuators 24 c and 24 d,respectively, and are movable in the direction of an optical axis.Spherical aberration correction of each recording layer of the opticaldisk 20 is realized by adjusting a distance between the first and secondcorrection lenses 24 a and 24 b. Laser beams corrected for sphericalaberration by the expander lens 24 are supplied to the objective lens25. The objective lens 25 causes the parallel laser beams to converge.The objective lens 25 has an actuator 25 a with a focusing part forcausing the objective lens 25 to move in the direction of the opticalaxis, and a tracking part for causing the objective lens 25 to move inthe direction of a disk radius that is perpendicular to the opticalaxis. The focusing part focuses a laser beam onto one of the recordinglayers of the optical disk 20. The tracking part places the light spotof the laser beam on the track of the one recording layer.

The laser beam reflected off any one of the recording layers of theoptical disk 20 returns as parallel laser beams to the beam splitter 23through the objective lens 25 and the expander lens 24. Then, the beamsplitter 23 causes reflection of the reflected laser beams at an angleof about 90 degrees with respect to the incidence, and supplies thereflected laser beams to the condenser lens 26. The condenser lens 26condenses the reflected laser beams on the light receiving surface ofthe detector 27 to form a spot on the light receiving surface. Thedetector 27 has a light receiving surface with four quadrants, forexample, and generates a voltage signal for each quadrant the level ofwhich is responsive to the intensity of received light.

The laser driving circuit 30 of the signal processing system drives thelight source 21 in response to instructions by the controller 32. Thesignal processing circuit 31 generates an RF signal that is a readsignal from recorded information in response to an output voltage signalof the detector 27. The signal processing circuit 31 also generatesservo signals such as a focus error signal and a tracking error signal.A focus error signal may be generated, for example, by a publicly knownsignal generating method such as an astigmatic method. A tracking errorsignal may be generated, for example, by a publicly known signalgenerating method such as a push-pull method.

The controller 32 receives servo signals from the signal processingcircuit 31. Then, the controller 32 supplies a tracking control signaland a focusing control signal to the objective lens driving circuit 33in order to realize tracking control and focusing control by theobjective lens 25. The controller 32 supplies a spherical aberrationcorrection control signal to the spherical aberration correcting elementdriving circuit 34 in order to realize spherical aberration correctioncontrol by the expander lens 24. A spherical aberration correctioncontrol signal to be generated is such that it indicates a sphericalaberration correction value optimum for a recording layer on whichfocusing is to be realized. More specifically, a spherical aberrationcorrection value optimum for each recording layer is stored as a datatable in the memory 35. A spherical aberration correction valuecorresponding to a recording layer on which focusing is to be realizedis extracted from the data table, and then a spherical aberrationcorrection control signal indicative of the extracted sphericalaberration correction value is generated. Generally, a tracking controlsignal is generated such that it makes the level of a tracking errorsignal zero, and a focusing control signal is generated such that itmakes the level of a focus error signal zero. The memory 35 stores anoperating program of the controller 32 and a data table.

The objective lens driving circuit 33 drives the tracking part of theactuator 25 a in response to a tracking control signal to cause theobjective lens 25 to move in the direction of the disk radius that isperpendicular to the optical axis. The objective lens driving circuit 33drives the focusing part of the actuator 25 a in response to a focusingcontrol signal to cause the objective lens 25 to move in the directionof the optical axis. The spherical aberration correcting element drivingcircuit 34 drives the actuators 24 c and 25 d in response to a sphericalaberration correction control signal to cause the first and secondcorrection lenses 24 a and 24 b to move in the direction of the opticalaxis.

An RF signal is demodulated at a demodulation processing circuit notshown to become an audio signal and an image signal.

A description will be given of the reason why multiple reflection isreduced by determining the amount of light reflected back to the surfaceof incidence of the optical disk to be smaller as a recording layer islocated farther from the surface of incidence. In the following, like inthe case of FIG. 1, the number of recording layers of the optical diskis eight, namely the optical disk has recording layers L0 to L7.

FIG. 7 is a diagram illustrating the amount of light reflected backafter entering the multilayer optical disk and after being reflected offeach of recording layers L_(N−1) and L_(N−2). In FIG. 7, Ri is theamount of light reflected back from an i layer, r_(i) is the reflectanceof the i layer, t_(i) is the transmittance of the i layer, r_(su) is asurface reflectance, t_(su) is a surface transmittance(r_(su)+t_(su)=1), t_(sp) is the transmittance of each intermediatelayer, and N is the number of recording layers.

The amount of light reflected back from the recording layer L_(N−1)(layer nearest the surface of incidence) is considered first. Forsimplicity, the amount of light that enters the disk is set at 1.0. Theamount of light passing through the surface of incidence of the disk ist_(su), and the transmittance of each intermediate layer is t_(sp).Accordingly, the amount of the light that reaches the recording layerL_(N−1) is expressed as t_(su)t_(sp). The reflectance of the recordinglayer L_(N−1) is r_(N−1). Accordingly, the amount of light in a stateimmediately after the light is reflected off the recording layer L_(N−1)is t_(su)t_(sp)r_(N−1). The amount of this light that again passedthrough an intermediate layer is t_(su)t_(sp) ²r_(N−1). The amount ofthe light R_(N−1) that finally returns from the disk after passingthrough the disk surface is expressed as R_(N−1)=t_(su) ²t_(sp)²r_(N−1). Likewise, the amount of light reflected back from therecording layer L_(N−2) is expressed as R_(N−2)=t_(su) ²t_(sp) ⁴t_(N−1)²r_(N−1). The amount of light reflected back from the recording layer Li(i=zero to N−1) is generally expressed as follows:

$\begin{matrix}{{{Ri} = {{T_{i}^{2} \times r_{i}} = {t_{su}^{2}{t_{sp}^{2{({N - i})}}\left( {\prod\limits_{j = {i + 1}}^{N - 1}t_{j}^{2}} \right)}}}}{{Herein},}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \\{{Ti} = {t_{su}{t_{sp}^{N - i}\left( {\prod\limits_{j = {i + 1}}^{N - 1}t_{j}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

indicates the amount of light that reaches the recording layer Li.

The conventional multilayer optical disk is designed such that theamounts of light reflected back from all recording layers are the same.More specifically, the optical disk is designed so thatR ₀ =R ₁ = . . . R _(N−1) =R _(ref)  [Formula 3]

is satisfied.

The reflectance r_(i) of each recording layer is obtained as followsfrom Formulas 1 and 3:

$r_{i} = {\frac{R_{ref}}{T_{i}^{2}} = \frac{R_{ref}}{t_{su}^{2}{t_{sp}^{2{({N - i})}}\left( {\prod\limits_{j = {i + 1}}^{N - 1}t_{j}^{2}} \right)}}}$

The reflectances of the respective layers of the eight-layer disk areshown in FIG. 8 that are calculated under conditions that r_(su)=4%(t_(su)=96%), t_(sp)=97.8%, and R_(ref)=5%.

As seen from FIG. 8, by making the amounts of light R_(i) reflected backfrom all the recording layers to be the same, the reflectance r_(i) of arecording layer becomes greater as the recording layer is locatedfarther from the surface of incidence. Thus, the following formula isestablished in the conventional multilayer optical disk:r ₀ >r ₁ > . . . r _(N−2) >r _(N−1)  [Formula 5]

A path of a light beam reflected back from each recording layer at thetime of playback of the recording layer L0 is shown in FIG. 9( a). Theamounts of light R₀, R₁, and R₂ reflected back from the recording layersL₀, L₁, and L₂ after being reflected once off the recording layers L₀,L₁, and L₂, respectively, are expressed as follows:R₀=T₂ ²t_(sp) ⁴t₂ ²t₁ ²r₀R₁=T₂ ²t_(sp) ²t₂ ²r₁R₂=T₂ ²r₂  [Formula 6]

The amount of light R₀ ^(1st) reflected back after being reflectedseveral times to form a first confocal spot is expressed as follows:R₀ ^(1st)=T₂ ²t_(sp) ⁴t₂ ²r₁ ²r₂  [Formula 7]

A degree of effect of multiple reflection CT during playback of therecording layer L0 is expressed as follows by using a ratio between R₀and R₀ ^(1st):

$\begin{matrix}{\frac{R_{0}^{1{st}}}{R_{0}} = {\frac{r_{1}^{2}r_{2}}{t_{1}^{2}r_{0}} = {{t_{sp}^{2}\left( \frac{R_{1}}{R_{0}} \right)}r_{1}r_{2}}}} & \left\lbrack {{Formula}\mspace{14mu} 8} \right\rbrack\end{matrix}$

A degree of effect of multiple reflection CT during playback of therecording layer L1 is determined in exactly the same manner, and isexpressed as follows by using a ratio between R₁ and R₁ ^(1st):

$\begin{matrix}{\frac{R_{1}^{1{st}}}{R_{1}} = {\frac{r_{2}^{2}r_{3}}{t_{2}^{2}r_{1}} = {{t_{sp}^{2}\left( \frac{R_{2}}{R_{1}} \right)}r_{2}r_{3}}}} & \left\lbrack {{Formula}\mspace{14mu} 9} \right\rbrack\end{matrix}$

Accordingly, a degree of effect of multiple reflection CT duringplayback of the recording layer Li is expressed as follows by using aratio between R_(i) and R_(i) ^(1st):

$\begin{matrix}{\frac{R_{i}^{1{st}}}{R_{i}} = {{t_{sp}^{2}\left( \frac{R_{i + 1}}{R_{i}} \right)}r_{i + 1}r_{i + 2}}} & \left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The following formula is established from Formulas 3, 5 and 10 givenabove:

$\begin{matrix}{\frac{R_{0}^{1{st}}}{R_{0}} > \frac{R_{1}^{1{st}}}{R_{1}} > \ldots > \frac{R_{N - 3}^{1{st}}}{R_{N - 3}}} & \left\lbrack {{Formula}\mspace{14mu} 11} \right\rbrack\end{matrix}$

In the conventional multilayer disk, the effect of multilayer reflectionCT is greater as a layer is located farther from the surface. The reasonwhy Formula 11 ends with N−3 is that a confocal spot caused by multiplereflection is not generated in the nearest recording layer L_(N−1) andthe recording layer L_(N−2) next to the recording layer L_(N−1).

It is seen from Formula 10 that a degree of effect of multiplereflection CT is reduced by reducing the reflectances r_(i+1) andr_(i+2) of the recording layers. It is also seen from Formula 4 thatr_(i+1) and r_(i+2) are reduced by reducing R_(ref). That is, a degreeof effect of multiple reflection CT becomes smaller by reducing theamount of light reflected back after being reflected once off arecording layer. However, reducing R_(ref) means lowering a signallevel. Accordingly, a signal S/N degrades if R_(ref) is excessivelyreduced.

A signal S/N in optical disk systems includes two types: electric S/Nand optical S/N. The former is a ratio between an electrical signal Seformed as a result of photoelectric conversion of signal light from arecording layer at a detector (photodetector) and an electrical noise Negenerated in an electrical circuit system (Ne is constant as it isdetermined by a circuit system), and is expressed as Se/Ne. The latteris a ratio between the intensity So of signal light reflected back froma recording surface and the intensity No of stray light reflected off asurface other than a recording surface, and is expressed as So/No.

The electrical signal Se is proportionate to the signal light intensitySo. The signal light intensity So is determined by a product of theamount of light R_(ref) reflected once and the amount of incident light.Accordingly, if R_(ref) is reduced to reduce the effect of multiplereflection CT, the amount of incident light may be increased tocompensate for the reduction of R_(ref), and a product of R_(ref) andthe amount of incident light may be stored. Thus, degradation of Se/Neis avoided.

The intensity No of stray light reflected off a surface other than arecording surface is proportionate to the amount of incident light.Accordingly, reducing R_(ref) to reduce the effect of multiplereflection CT, and increasing the amount of incident light to compensatefor the reduction of R_(ref) result in degradation of So/No.

Stray light reflected off a surface other than a recording surface mayinclude interface reflected light generated on a disk surface, and straylight reflected off an optical member such as an objective lens.However, the stray light intensity No is hardly influenced as anantireflection film to prevent stray light reflection is provided togenerally employed optical members, or an optical axis is slightlytilted to prevent reflection from reaching a detector. Accordingly, themost significant No component in an optical disk system is surfacereflecting light on a disk.

DVDs and BDs as optical disks have one or two recording layers and donot have a confocal spot accordingly, so that these optical disks do notsuffer from multiple reflection CT. Accordingly, R_(ref) has beendesigned to have a value close to its maximum possible value. Thus, theintensity No of light reflected from the disk surface is sufficientlysmall relative to the intensity So of light reflected from a recordinglayer, and does not cause any adverse effect. However, in the case of amultilayer disk with three or more layers, surface reflecting light onthe disk becomes nonnegligible as a possible value of R_(ref)significantly decreases, and reflectance may be reduced further tosuppress multiple reflection CT.

As described above, it is hard to reduce multiple reflection CT byreducing R_(ref) if the stray right intensity No is nonnegligible.Accordingly, the conventional techniques disclosed in Patent Literatures1 to 4 listed above try to prevent generation of a confocal spot bycontrolling an interlayer distance.

However, as already described in “Problem to be Solved by theInvention”, strictly controlling an interlayer distance results invarious disadvantages in manufacture of disks. The invention has beenmade in light of this, and is intended to reduce the effect of multiplereflection CT while causing no reduction in S/N without strictlycontrolling an interlayer distance.

FIG. 10 is a graph showing the amount of surface reflecting light thatenters a detector with a horizontal axis indicating a focus position(distance from a disk surface). In this graph, surface reflectance is4%, and the size of a standardized detector (determined by dividing adetector area by the square of a magnification ratio) is 44 [μm²].

It is seen from FIG. 10 that the surface reflecting light entering thedetector significantly decreases as a focus position gets farther fromthe disk surface. That is, while being nonnegligible during playback ofa recording layer near the surface of incidence of the disk, the straylight intensity No is sufficiently small during playback of a recordinglayer located farther from the surface of incidence. This means that,while R_(ref) cannot be reduced much to reduce the effect of multiplereflection CT in the case of a recording layer near the surface ofincidence, R_(ref) can be reduced sufficiently to reduce the effect ofmultiple reflection CT in the case of a recording layer located fartherfrom the surface of incidence.

Accordingly, as shown in FIG. 3, the structure of the multilayer opticaldisk of the present invention is configured such that the amount ofreflected light R_(i) gradually decreases in the order from a recordinglayer near the surface of incidence of the optical disk to a recordinglayer located farther from the surface of incidence.

A description by using Formulas will be given of how multiple reflectionCT can be reduced by the multilayer optical disk of the presentinvention. For simplicity, the following description refers to theeffect of multiple reflection CT generated by a first confocal spotduring playback of the recording layer L0 of the optical disk with 8recording layers farthest from the surface of incidence.

Formula 8 given above indicating a degree of effect of multiplereflection CT during playback of the recording layer L0 may be modifiedas follows:

$\begin{matrix}{\frac{R_{0}^{1{st}}}{R_{0}} = {{{t_{sp}^{2}\left( \frac{R_{1}}{R_{0}} \right)}r_{r}r_{2}} = {\left( \frac{R_{2}}{R_{0}} \right)\left( \frac{R_{1}}{T_{2}^{2}t_{2}} \right)^{2}}}} & \left\lbrack {{Formula}\mspace{14mu} 12} \right\rbrack\end{matrix}$

Relationship ri+ti=1 is established if absorption at a recording layeris sufficiently small. Accordingly, Formula 12 is also modified forsimplicity by using this relationship and employing Formula 2, therebyestablishing formula as follows:

$\begin{matrix}\begin{matrix}{\frac{R_{0}^{1{st}}}{R_{0}} = {\left( \frac{R_{2}}{R_{0}} \right)\left( \frac{R_{1}}{T_{2}^{2}t_{2}} \right)^{2}}} \\{= {\left( \frac{R_{2}}{R_{0}} \right)\left\{ \frac{R_{1}}{T_{2}^{2}\left( {1 - r_{2}} \right)} \right\}^{2}}} \\{= {\left( \frac{R_{2}}{R_{0}} \right)\left( \frac{R_{1}}{T_{2}^{2} - R_{2}} \right)^{2}}}\end{matrix} & \left\lbrack {{Formula}\mspace{14mu} 13} \right\rbrack\end{matrix}$

Formula 3 is satisfied in the conventional multilayer disk. Accordingly,Formula 13 is also expressed as follows:

$\begin{matrix}{{\frac{R_{0}^{1{st}}}{R_{0}} = \left( \frac{R_{ref}}{T_{2}^{2} - R_{ref}} \right)^{2}},{T_{2} = {t_{su}t_{sp}^{6}{\prod\limits_{j = 3}^{7}\left( {1 - \frac{R_{ref}}{T_{j}^{2}}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 14} \right\rbrack\end{matrix}$

In contrast, relationship R₀<R₁< . . . R_(N−1)=R_(ref) is established inthe optical disk of the present invention. The following relationshipsare also provided for simplicity:R₇=R_(ref)R₆=αR₇R₅=αR₆=α²R_(ref)R₄=αR₅=α³R_(ref)R₃=αR₄=α⁴R_(ref)R₂=αR₃=α⁵R_(ref)R₁=αR₂=α⁶R_(ref)R ₀ =αR ₁=α⁷ R _(ref)(a<1)  [Formula 15]

Substituting Formula 15 into Formula 13 results in the following:

$\begin{matrix}{{\frac{R_{0}^{1{st}}}{R_{0}} = {\alpha^{10}\left( \frac{R_{ref}}{T_{2}^{2} - {\alpha^{5}R_{ref}}} \right)}^{2}},{T_{2}^{\prime} = {t_{su}t_{sp}^{6}{\prod\limits_{j = 3}^{7}\left( {1 - \frac{R_{j}}{T_{j}^{2}}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 16} \right\rbrack\end{matrix}$

Relationship T₂′>T₂ is established between the amounts of light T₂ andT₂′ reaching the recording the recording layer L2 that are determined byFormulas 14 and 16 respectively, meaning that more light reaches arecording layer located farther from the surface of incidence in theoptical disk of the present invention. This produces a side benefit asr_(i) can be made smaller than that of the conventional optical disk inorder to obtain R_(i) of the same value. As a result, an effect producedby reducing R_(i) increases exponentially as a recording layer islocated farther from the surface of incidence.

A ratio between Formulas 14 and 16 is determined as follows in order tocompare the respective degrees of effects of multiple reflection CT inthe conventional optical disk and the optical disk of the presentinvention:

$\begin{matrix}{\frac{{Formula}\mspace{14mu} 16\left( {{optical}\mspace{14mu}{disk}\mspace{14mu}{of}\mspace{14mu}{invention}} \right)}{{Formula}\mspace{14mu} 14\left( {{conventional}\mspace{14mu}{optical}\mspace{14mu}{disk}} \right)} = {\alpha^{10}\left( \frac{T_{2}^{2} - R_{ref}}{T_{2}^{\prime 2} - {\alpha^{5}R_{ref}}} \right)}^{2}} & \left\lbrack {{Formula}\mspace{14mu} 17} \right\rbrack\end{matrix}$

Accordingly, on condition that a result of Formula 17 is smaller thanone, the effect of multiple reflection CT is reduced by establishingrelationship R₀<R₁< . . . R_(N−1). As α¹⁰<1 is obviously knownrelationship, a result of Formula 17 is smaller than one as long as thefollowing formula is established:

$\begin{matrix}{\left( \frac{T_{2}^{2} - R_{ref}}{T_{2}^{2} - {\alpha^{5}R_{ref}}} \right) < 1} & \left\lbrack {{Formula}\mspace{14mu} 18} \right\rbrack\end{matrix}$

As already described, T₂′ is greater than T₂, and α¹⁰ is smaller thanone. Accordingly, it is obviously known that the denominator is greaterthan the numerator of Formula 17. As a result, it is obviously seen thatthe optical disk of the present invention reduces the effect of multiplereflection CT.

For simplicity of the description, each relationship given in Formula 15is employed to show that the amount of light reflected back from arecording layer is smaller as the recording layer is located fartherfrom the surface of incidence. However, not all the relationships inFormula 15 are required to be satisfied. As an example, if the amount ofreflected light Ri linearly decreases in the order from a recordinglayer nearest the surface of incidence of an optical disk to a recordinglayer located farthest from the surface of incidence as shown in FIG.11, the ratio R_(i) ^(1st)/R_(i) indicating a degree of effect ofmultiple reflection CT can be made smaller than that of the conventionaltechnique as shown in FIG. 12.

As described above, use of the multilayer optical disk of the presentinvention suppresses effect of multiple reflection generated on arecording layer located far from a surface of incidence, and suppresseseffect of surface reflection that causes adverse effect during playbackof a recording layer near the surface of incidence. Use of themultilayer optical disk of the present invention also reduces multiplereflection CT without positively changing adjacent interlayer distances.This increases yield in manufacture of disks, while making any specificcontrol logic unnecessary in a drive device.

FIG. 13 shows respective values of jitter generated during playback ofthe conventional 20-layer optical disk designed by the conventionaltechnique such that the amounts of light reflected back from recordinglayers are uniformly 1.2% with respect to the amount of incident light,and of the 20-layer optical disk of the present invention. The 20-layeroptical disk of the present invention mainly reduces multiple reflectionCT generated during playback of a recording layer located farther fromthe surface of incidence. Accordingly, a jitter value is improved on therecording layer located farther from the surface of incidence.

FIG. 14 shows an exemplary reduction rate of a jitter value of therecording layer located farthest from the surface of incidence of anoptical disk (calculated by dividing the jitter value of the farthestrecording layer of the optical disk of the present invention by thejitter value of the farthest recording layer of the conventional opticaldisk). It is seen from FIG. 14 that a jitter value decreases to agreater extent with the number of recording layers increasing. Thecharacteristics of an attenuation rate shown in FIG. 14 are obtainedunder the following conditions. For the conventional optical disk,distances between recording layers are uniformly 10 μm, and the amountsof light reflected back from all the recording layers are uniform. Forthe optical disk of the present invention, distances between recordinglayers are uniformly 10 μm, the amount of light reflected back from thefarthest recording layer is half the amount of light reflected back fromthe nearest recording layer, and the amount of reflected light islinearly attenuated toward the farthest recording layer.

Assuming that the refractive index of a cover layer or an intermediatelayer of an optical disk is 1.5, the disk surface has a reflectance ofabout 4%. Meanwhile, a recording layer nearest the surface of incidenceof a 20-layer optical disk is spaced about 15 μm from the surface. Inlight of the characteristics of the relationship shown in FIG. 10between a focus position and surface reflecting light entering adetector, it is seen that the surface reflecting light entering thedetector during playback of the recording layer L19 nearest the surfaceof incidence is about 0.1%. In order for So/No mentioned above to be atleast 20 dB or more, the amount of light R₁₉ reflected back from therecording layer L19 should be at least ten times or more the surfacereflection light, namely 1% or more.

It is assumed that the amount of light R₁₉ reflected back from therecording layer L19 is from 1.0 to 1.6%, and that the amount ofreflected light linearly decreases in the order from the recording layerL19 toward recording layers located farther from the recording layerL19. In this case, in order to reduce multiple reflection CT to asufficient level on the farthest recording layer L0, the amount of lightR₀ reflected back from the farthest recording layer L0 should bereduced. FIG. 15 shows to which degree the amount of light R₀ reflectedback from the farthest recording layer L0 should be reduced.

It is seen from FIG. 15 that increasing the amount of reflected lightR₁₉ requires more reduction of the amount of reflected light R₀ in orderto achieve the same effect. Reducing the amount of reflected light R₀increases the amount of incident light required to maintain Se/Ne at thesame level. Increasing the amount of incident light results not only ingreater power consumption but also in the need of enhancing lightresistance of a disk (as the disk deteriorates as a result of repeatedapplication of a laser beam). Accordingly, it is desired that the amountof reflected light R₀ be not reduced excessively.

In light of the foregoing, the above-mentioned 20-layer optical disk ofExample is designed such that the reflectance of the recording layer L19is 1.2%, the reflectance of the farthest recording layer L0 is 0.6%(namely, the reflectance making the amount of light reflected backtherefrom half the amount of light reflected back from the recordinglayer L19), and a reflectance linearly decreases for the recordinglayers therebetween.

It is also seen from FIG. 15 that the jitter value of the farthestrecording layer L0 can be 10% or less only on the condition that thevalue of R₁₉/R₀ is 0.8 or lower. It cannot be concluded that jitter of10% is an appropriate condition of system establishment as theperformance of a recording/playback drive should be considered. However,it is seen that relationship R₁₉/R₀≦0.8 should be satisfied in order toachieve an effect from the viewpoint of a jitter value as an index.

Linearly reducing a reflectance results in an advantage in that eachrecording layer becomes free from excessive increase of multiplereflection CT from its adjacent layer. Meanwhile, it is desirable thatthe respective amounts of light reflected back from the recording layersL19 and L0 be made large and small. Thus, a distribution with suddenchange such as that shown in FIG. 16 is conceivable, for example. In thedistribution of FIG. 16, the amounts of light reflected back from therecording layers L19 to L9 are uniformly 1.2%, and those reflected backfrom the recording layers L8 to L0 are uniformly 0.6% This distributionof the amount of reflected light may in fact reduce the effect ofmultiple reflection CT. However, this distribution in turn relativelyincreases interlayer crosstalk light (once reflected light) from therecording layer L9 that is generally generated during playback of therecording layer L8 or L7. Accordingly, the aforementioned distributionof the amount of reflected light with sudden change is not desirable.

The intermediate layers between the recording layers have the samethickness in the optical disk of Example described above. As shown inFIG. 17, the intermediate layers may have two types of thicknesses (LAand LB), and may be arranged in a structure with alternating layers inwhich the intermediate layers of the different thicknesses arealternately arranged.

A multilayer optical disk of a structure with alternating layers reducesthe number of confocal spots by half generated as a result of multiplereflection, so that the effect of multiple reflection CT is smaller thanthat of a disk of a structure with uniform layers. However, as aconfocal spot is generated once every two layers, even the structurewith alternating layers may still suffer from the effect of multiplereflection CT depending on the reflectance of each layer. In this case,by introducing the design concept of the present application, thereflectance of each layer is designed such that the amount of reflectedlight decreases in the order from the nearer side to the farther side.This effectively reduces the effect of multiple reflection CT.

If the alternating layers are employed to reduce the effect of multiplereflection CT, a difference between the smaller and greater interlayerdistances LA and LB is preferably equal to or greater than 4 μm in lightof an error in manufacture of disks, the depth of focus of a playbackbeam spot, and the like.

The optical disk shown in FIG. 17 was described to have a structure withalternating layers in which interlayer distances are smaller and greaterthan an interlayer distance of a structure with uniform layers in orderto maintain a distance between the nearest and farthest layers at thesame level. However, reducing an interlayer distance increases generallyoccurring interlayer CT (once reflected CT). Accordingly, an actualsituation is that the smaller interlayer should be the same as that ofthe structure of uniform layers, and that the other interlayer should beincreased. However, an increased distance between the nearest andfarthest layers necessitates extension of the dynamic range of aspherical aberration correcting optical system. Accordingly,restrictions are also imposed to keep increase of this distance at theminimum possible level.

In light of the foregoing, an appropriate difference between two typesof interlayer distances is about 4 to 6 μm if a structure withalternating layers is employed.

The optical disk as an optical recording medium described above inExample has eight or 20 stacked recording layers. The present inventionis also applicable to an optical disk with three or more stackedrecording layers. While the optical disk described above is aplayback-only optical disk with recording layers on which pits aretransferred. The present invention is also applicable to a write-once orrewritable optical disk. The shape of an optical recording medium is notnecessarily a disk. An optical memory with three or more stackedrecording layers is also applicable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a path of light reflection when a laser beamis focused on the farthest recording layer of a multilayer optical disk.

FIG. 2 is a cross sectional view of Example of the present invention.

FIG. 3 is a diagram showing the reflectance of each recording layer andthe amount of light reflected back from each recording layer of theoptical disk of FIG. 2.

FIG. 4 is a diagram showing the relationship between the thickness of adielectric reflective film and a reflectance.

FIG. 5 is a diagram showing a method of manufacturing the optical diskof FIG. 2.

FIG. 6 is a diagram showing the structure of a disk drive device foractuating the optical disk of FIG. 2.

FIG. 7 is a diagram illustrating the amount of light reflected backafter entering a multilayer optical disk and after being reflected ofeach recording layer.

FIG. 8 is a diagram showing the reflectance of each recording layer.

FIG. 9 is a diagram showing a path of a light beam reflected back fromeach recording layer during playback.

FIG. 10 is a diagram showing the amount of surface reflecting lightentering a detector with respect to a focus position.

FIG. 11 is a diagram showing a difference in the amount of reflectedlight between a conventional optical disk and the optical disk of thepresent invention.

FIG. 12 is a diagram showing a difference in a degree of effect ofmultiple reflection CT between the conventional optical disk and theoptical disk of the present invention.

FIG. 13 is a diagram showing reduction of a jitter value in the disk ofFIG. 2.

FIG. 14 is a diagram showing the attenuation rate of the jitter value ofthe farthest recording layer.

FIG. 15 is a diagram showing a condition to make the jitter value of thefarthest recording layer to be 10% or less.

FIG. 16 is a diagram showing a distribution of the amount of reflectedlight controlled to have two values.

FIG. 17 is a diagram showing a multilayer optical disk of a structurewith alternating layers.

[EXPLANATION OF REFERENCE SIGNS] L0 to L19 Recording layer 20 Opticaldisk 21 Light source 23 Beam splitter 24 Expander lens 25 Objective lens27 Detector 31 Signal processing circuit 32 Controller 33 Objective lensdriving circuit 34 Spherical aberration correcting element drivingcircuit

The invention claimed is:
 1. A multilayer optical recording medium withat least three stacked recording layers, wherein, as a recording layeris located farther from a surface of incidence of a light beam forreading, an amount of light that reaches the surface of incidence afterbeing reflected off the recording layer is smaller; and wherein theamount of light reflected back from the recording layer located farthestfrom the surface of incidence is 0.5 times or more the amount of lightreflected back from the recording layer located nearest the surface ofincidence.
 2. The optical recording medium according to claim 1, whereinan amount of light reflected back from a recording layer locatedfarthest from the surface of incidence is 0.8 times or less an amount oflight reflected back from a recording layer located nearest the surfaceof incidence.
 3. The optical recording medium according to claim 1,wherein the amount of reflected light linearly decreases in the orderfrom the recording layer located nearest the surface of incidence to therecording layer located farthest from the surface of incidence.
 4. Amultilayer optical recording medium with at least three stackedrecording layers, wherein, as a recording layer is located farther froma surface of incidence of a light beam for reading, an amount of lightthat reaches the surface of incidence after being reflected off therecording layer is smaller, wherein different interlayer distances arealternately employed between adjacent recording layers.
 5. The opticalrecording medium according to claim 4, wherein a distance between thedifferent interlayer distances is four to six microns.
 6. A multilayeroptical recording medium with at least three stacked recording layers,wherein, as a recording layer is located farther from a surface ofincidence of a light beam for reading, an amount of light that reachesthe surface of incidence after being reflected off the recording layeris smaller, wherein an amount of light reflected back from a recordinglayer located farthest from the surface of incidence is 0.8 times orless an amount of light reflected back from a recording layer locatednearest the surface of incidence, and wherein different interlayerdistances are alternately employed between adjacent recording layers. 7.The optical recording medium according to claim 6, wherein a distancebetween the different interlayer distances is four to six microns.
 8. Amultilayer optical recording medium with at least three stackedrecording layers, wherein, as a recording layer is located farther froma surface of incidence of a light beam for reading, an amount of lightthat reaches the surface of incidence after being reflected off therecording layer is smaller, wherein the amount of reflected lightlinearly decreases in the order from the recording layer located nearestthe surface of incidence to the recording layer located farthest fromthe surface of incidence, and wherein different interlayer distances arealternately employed between adjacent recording layers.
 9. The opticalrecording medium according to claim 8, wherein a distance between thedifferent interlayer distances is four to six microns.